In magneto-optical drives, a magnetic field of a minimum strength is required at the focal point of an objective lens in order to perform the operations of writing or erasing data. The magnetic field was created by passing constant current through bias coils of finite length, thus creating non-uniform magnetic fields. This non-uniformity leads to a problem. First, if a magnetic field was created allowing writing or erasing data at all positions of the focal point, excess power was dissipated. Second, if excess power was minimized, there would be positions of the focal point where the magnetic field was insufficient to allow writing or erasing data.
Furthermore, the bias coil was not the only source of magnetic fields at the focal point of the objective lens. Other sources existed, such as the focus actuator and the spindle motor. Because prior art systems used a constant current, sources which created uniform magnetic fields could be accounted for. However, non-uniform magnetic fields could not be accounted for accurately, leading to the same problems listed above.